U.S. patent application number 12/242204 was filed with the patent office on 2010-04-01 for detecting a signal quality decrease in a measurement system.
This patent application is currently assigned to Nellcor Puritan Bennett Ireland. Invention is credited to Paul Stanley Addison, James Nicholas Watson.
Application Number | 20100079279 12/242204 |
Document ID | / |
Family ID | 41507915 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100079279 |
Kind Code |
A1 |
Watson; James Nicholas ; et
al. |
April 1, 2010 |
Detecting a Signal Quality Decrease in a Measurement System
Abstract
Techniques for detecting a signal quality decrease are
disclosed. A sensor or probe may be used to obtain a plethysmograph
or photoplethysmograph (PPG) signal from a subject. A wavelet
transform of the signal may be performed and a scalogram may be
generated based at least in part on the wavelet transform. One or
more characteristics of the scalogram may be determined. The
determined characteristics may include, for example, energy values
and energy structural characteristics in a pulse band, a mains hum
band, and/or a noise band. Such characteristics may be analyzed to
produce signal quality values and associated signal quality trends.
One or more signal quality values and signal quality trends may be
used to determine if a signal quality decrease has occurred or is
likely to occur.
Inventors: |
Watson; James Nicholas;
(Dunfermline, GB) ; Addison; Paul Stanley;
(Edinburgh, GB) |
Correspondence
Address: |
Nellcor Puritan Bennett LLC;ATTN: IP Legal
6135 Gunbarrel Avenue
Boulder
CO
80301
US
|
Assignee: |
Nellcor Puritan Bennett
Ireland
Galway
IE
|
Family ID: |
41507915 |
Appl. No.: |
12/242204 |
Filed: |
September 30, 2008 |
Current U.S.
Class: |
340/540 |
Current CPC
Class: |
A61B 5/14551 20130101;
A61B 5/7221 20130101; A61B 5/6843 20130101; G08B 21/18 20130101;
A61B 5/726 20130101 |
Class at
Publication: |
340/540 |
International
Class: |
G08B 21/00 20060101
G08B021/00 |
Claims
1. A method of detecting a signal quality decrease event
comprising: obtaining a signal; computing a continuous wavelet
transform of the signal; generating a scalogram based at least in
part on the wavelet transform; analyzing one or more
characteristics of the scalogram; and determining whether a signal
quality decrease event has occurred based at least in part on the
analysis.
2. The method of claim 1, wherein obtaining the signal comprises
obtaining a PPG signal.
3. The method of claim 1, further comprising triggering a signal if
the signal quality decrease event is determined to have
occurred.
4. The method of claim 1, wherein the one or more characteristics
comprise characteristics selected from the group consisting of:
energy measures in the pulse band; energy measures in the mains hum
band; and energy measures in the noise band.
5. The method of claim 1, wherein the one or more characteristics
comprise characteristics selected from the group consisting of:
energy structure in the pulse band; energy structure in the mains
hum band; and energy structure in the noise band.
6. The method of claim 1, wherein analyzing the one or more
characteristics comprises determining one or more signal-to-noise
levels related to the one or more characteristics.
7. The method of claim 1, wherein analyzing the one or more
characteristics comprises curve-fitting one or more plots related
to the one or more characteristics.
8. The method of claim 1, wherein analyzing the one or more
characteristics comprises weighing one or more signal qualities and
one or more associated signal quality trends to determine an
overall signal quality and an associated overall signal quality
trend.
9. The method of claim 1, wherein the one or more characteristics
are determined by applying one or more time-windows to the
scalogram.
10. A system for detecting a signal quality decrease event
comprising: a sensor capable of obtaining a signal; a processor
capable of: computing a continuous wavelet transform of the signal;
generating a scalogram based at least in part on the wavelet
transform; analyzing one or more characteristics of the scalogram;
and determining whether a signal quality decrease event has
occurred based at least in part on the analysis.
11. The system of claim 10, wherein the sensor is capable of
obtaining a PPG signal.
12. The system of claim 10, wherein the processor is capable of
triggering a signal if the signal quality decrease event is
determined to have occurred.
13. The system of claim 10, wherein the one or more characteristics
comprise characteristics selected from the group consisting of:
energy measures in the pulse band; energy measures in the mains hum
band; and energy measures in the noise band.
14. The system of claim 10, wherein the one or more characteristics
comprise characteristics selected from the group consisting of:
energy structure in the pulse band; energy structure in the mains
hum band; and energy structure in the noise band.
15. The system of claim 10, wherein the processor is capable of
analyzing the one or more characteristics by determining one or
more signal-to-noise levels related to the one or more
characteristics.
16. The system of claim 10, wherein the processor is capable of
analyzing the one or more characteristics by curve-fitting one or
more plots related to the one or more characteristics.
17. The system of claim 10, wherein the processor is capable of
analyzing the one or more characteristics by weighing one or more
signal qualities and one or more associated signal quality trends
to determine an overall signal quality and an associated overall
signal quality trend.
18. The system of claim 10, wherein the processor is capable of
determining the one or more characteristics by applying one or more
time-windows to the scalogram.
19. A computer-readable medium for use in detecting a signal
quality decrease event, the computer-readable medium having
computer program instructions recorded thereon for: obtaining a
signal; computing a continuous wavelet transform of the signal;
generating a scalogram based at least in part on the wavelet
transform; analyzing one or more characteristics of the scalogram;
and determining whether a signal quality decrease event has
occurred based at least in part on the analysis.
20. The computer-readable medium of claim 19, having further
computer program instructions for analyzing the one or more
characteristics by weighing one or more signal qualities and one or
more associated signal quality trends to determine an overall
signal quality and an associated overall signal quality trend.
Description
SUMMARY
[0001] The present disclosure relates to signal processing and,
more particularly, the present disclosure relates to using
characteristics of one or more wavelet scalograms of a signal, such
as a photoplethysmograph (PPG) signal, to determine if a signal
quality decrease has occurred or is likely to occur in a system,
such as a pulse oximetry system.
[0002] In an embodiment, a pulse oximetry system is used to measure
and analyze physiological characteristics of a patient. A signal
quality decrease may occur when the target stimulus (e.g., a
patient fingertip, toe, forehead, earlobe, or foot) is no longer
adequately reflected in measurement of the PPG signal. Possible
causes of such a signal quality decrease include motion artifacts
that may be caused by, for example, voluntary or involuntary
respiration, eye movements, swallowing, yawning, cardiac motion,
and/or general body movement of a patient, the sensor being
accidentally dislodged, or the sensor or any constituent component
of the sensor being damaged or otherwise malfunctioning.
[0003] In an embodiment, a PPG signal is transformed using a
continuous wavelet transform. Continuous wavelet transforms allow
for the use of multiple wavelets that are each scaled in accordance
with scales of interest of a signal such that smaller scale
components of a signal are transformed using wavelets scaled more
compactly than wavelets used to extract larger scale components of
the signal. The window size of data to which each wavelet gets
applied varies according to scale as well. Thus, a higher
resolution transform is possible using continuous wavelets relative
to discrete techniques.
[0004] In an embodiment, one or more scalograms may be obtained by
processing the wavelet transform. Each scalogram may represent the
energy density of the PPG signal, where a suitable scaling has been
performed to emphasize certain scale values or ranges of interest
for the analysis of the PPG signal. In addition, the scalogram may
contain information on the real part of the wavelet transform, the
imaginary part of the wavelet transform, the phase of the wavelet
transform, any other suitable part of the wavelet transform, or any
combination thereof.
[0005] In an embodiment, a set of one or more characteristics may
be determined by applying one or more time-windows to one or more
scalograms of a PPG signal. Time-windows may be continuous or
discontinuous, and may be used to isolate regions or scale bands of
the one or more scalograms. The characteristics that are determined
may chosen based on a pre-existing knowledge of features that are
expected in a scalogram before and after a signal quality decrease
event. For example, the characteristics that are determined may
include some or all of the following items: energy levels in a
pulse band, energy levels in a mains hum band; energy levels in a
noise band, energy structure in the pulse band, energy structure in
the mains hum band, and energy structure in the noise band.
[0006] In an embodiment, a set of one or more characteristics
derived from one or more scalograms may be analyzed. An analysis
may include curve-fitting one or more plots of data related to the
one or more characteristics. Plots of data may depict the average
or maximum energy observed in a given region of the scalogram as a
function of time) and curve-fitting may involve interpolating or
least-squares fitting the plotted data. An analysis may include
calculating one or more signal-to-noise levels based on the data
related to the one or more characteristics. The signal-to-noise
levels may correspond to the ratio of average or maximum energy
density in a suitable signal band, such as the pulse band, to the
average or maximum energy density in a suitable non-signal band,
such as the mains hum band or the noise band.
[0007] In an embodiment, one or more signal qualities and one or
more associated signal quality trends may be determined based on an
analysis of characteristics derived from one or more scalograms.
Each signal quality value may be represented by a number from 0 to
100, where a larger number indicates a higher quality signal, and
each signal quality trend may be represented by a number
representing a rate of increase or a rate of decrease in the signal
quality value versus time. One or more signal qualities and one or
more associated signal quality trends may be combined or weighed
according to any suitable method to determine an overall signal
quality value and an associated overall signal quality trend
value.
[0008] In an embodiment, an overall signal quality value and an
associated overall signal quality trend value may be used to
determine or anticipate the presence of a signal quality value
decrease event. A signal may be triggered if it is determined that
a signal quality decrease event has occurred or that one is likely
to occur. For example, the triggered signal may sound an alarm or
display one or more on-screen messages to alert a user of the
signal quality decrease. If it is determined that the signal
quality decrease event has not occurred, then a new portion of a
scalogram may be analyzed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0010] The above and other features of the present disclosure, its
nature and various advantages will be more apparent upon
consideration of the following detailed description, taken in
conjunction with the accompanying drawings in which:
[0011] FIG. 1 shows an illustrative pulse oximetry system in
accordance with an embodiment;
[0012] FIG. 2 is a block diagram of the illustrative pulse oximetry
system of FIG. 1 coupled to a patient in accordance with an
embodiment;
[0013] FIGS. 3(a) and 3(b) show illustrative views of a scalogram
derived from a PPG signal in accordance with an embodiment;
[0014] FIG. 3(c) shows an illustrative scalogram derived from a
signal containing two pertinent components in accordance with an
embodiment;
[0015] FIG. 3(d) shows an illustrative schematic of signals
associated with a ridge in FIG. 3(c) and illustrative schematics of
a further wavelet decomposition of these newly derived signals in
accordance with an embodiment;
[0016] FIGS. 3(e) and 3(f) are flow charts of illustrative steps
involved in performing an inverse continuous wavelet transform in
accordance with embodiments;
[0017] FIG. 4 is a block diagram of an illustrative continuous
wavelet processing system in accordance with an embodiment;
[0018] FIG. 5 shows an illustrative plot of a PPG signal taken
during a period of decreasing signal quality in accordance with an
embodiment;
[0019] FIG. 6 shows an illustrative scalogram derived from the PPG
signal of FIG. 5 in accordance with an embodiment;
[0020] FIG. 7 shows illustrative plots of an energy measure versus
time derived from the scalogram of FIG. 6 in accordance with an
embodiment;
[0021] FIG. 8 shows illustrative plots of the signal-to-noise level
versus time derived from FIG. 7 in accordance with an
embodiment;
[0022] FIG. 9 shows an illustrative plot of a PPG signal taken
prior to and during a period which includes a motion artifact in
accordance with an embodiment;
[0023] FIG. 10 shows an illustrative scalogram derived from the PPG
signal of FIG. 9 in accordance with an embodiment;
[0024] FIG. 11 is a flow chair of an illustrative process for
determining and responding to a decrease in signal quality in
accordance with an embodiment;
[0025] FIG. 12 shows a flow chart of an illustrative process for
determining a set of characteristics in accordance with FIG. 11 and
in an embodiment.
[0026] FIG. 13 shows a flow chart of an illustrative process for
analyzing the set of characteristics in accordance with FIG. 11 and
in an embodiment.
DETAILED DESCRIPTION
[0027] An oximeter is a medical device that may determine the
oxygen saturation of the blood. One common type of oximeter is a
pulse oximeter, which may indirectly measure the oxygen saturation
of a patients blood (as opposed to measuring oxygen saturation
directly by analyzing a blood sample taken from the patient) and
changes in blood volume in the skin. Ancillary to the blood oxygen
saturation measurement, pulse oximeters may also be used to measure
the pulse rate of the patient. Pulse oximeters typically measure
and display various blood flow characteristics including, but not
limited to, the oxygen saturation of hemoglobin in arterial
blood.
[0028] An oximeter may include a light sensor that is placed at a
site on a patient, typically a fingertip, toe, forehead or earlobe,
or in the case of a neonate) across a foot. The oximeter may pass
light using a light source through blood perfused tissue and
photoelectrically sense the absorption of light in the tissue. For
example, the oximeter may measure the intensity of light that is
received at the light sensor as a function of time. A signal
representing light intensity versus time or a mathematical
manipulation of this signal (e.g., a scaled version thereof, a log
taken thereof, a scaled version of a log taken thereof, etc.) may
be referred to as the photoplethysmograph (PPG) signal. In
addition, the term "PPG signal," as used herein, may also refer to
an absorption signal (i.e., representing the amount of Light
absorbed by the tissue) or any suitable mathematical manipulation
thereof. The light intensity or the amount of light absorbed may
then be used to calculate the amount of the blood constituent
(e.g., oxyhemoglobin) being measured as well as the pulse rate and
when each individual pulse occurs.
[0029] The light passed through the tissue is selected to be of one
or more wavelengths that are absorbed by the blood in an amount
representative of the amount of the blood constituent present in
the blood. The amount of light passed through the tissue varies in
accordance with the changing amount of blood constituent in the
tissue and the related light absorption. Red and infrared
wavelengths may be used because it has been observed that highly
oxygenated blood will absorb relatively less red light and more
infrared light than blood with a lower oxygen saturation. By
comparing the intensities of two wavelengths at different points in
the pulse cycle, it is possible to estimate the blood oxygen
saturation of hemoglobin in arterial blood.
[0030] When the measured blood parameter is the oxygen saturation
of hemoglobin, a convenient starting point assumes a saturation
calculation based on Lambert-Beer's law. The following notation
will be used herein:
I(.lamda.,t)=I.sub.o(.lamda.)exp(-(s.beta..sub.o(.lamda.)+(1-s).beta..su-
b.r(.lamda.))l(t)) (1)
where: [0031] .lamda.=wavelength; [0032] t=time; [0033] I=intensity
of light detected; [0034] I.sub.o=intensity of light transmitted;
[0035] s=oxygen saturation; [0036] .beta..sub.o,
.beta..sub.r=empirically derived absorption coefficients; and
[0037] l(t)=a combination of concentration and path length from
emitter to detector as a function of time.
[0038] The traditional approach measures light absorption at two
wavelengths (e.g., red and infrared (IR)), and then calculates
saturation by solving for the "ratio of ratios" as follows. [0039]
1. First, the natural logarithm of (1) is taken ("log" will be used
to represent the natural logarithm) for IR and Red
[0039] log I=log I.sub.o-(s.beta..sub.o+(1-s).beta..sub.r)l (2)
[0040] 2. (2) is then differentiated with respect to time
[0040] log I t = - ( s .beta. o + ( 1 - s ) .beta. r ) l t ( 3 )
##EQU00001## [0041] 3. Red (3) is divided by IR (3)
[0041] log I ( .lamda. R ) / t log I ( .lamda. IR ) / t = s .beta.
o ( .lamda. R ) + ( 1 - s ) .beta. r ( .lamda. R ) s .beta. o (
.lamda. IR ) + ( 1 - s ) .beta. r ( .lamda. IR ) ( 4 ) ##EQU00002##
[0042] 4. Solving for s
[0042] s = log I ( .lamda. IR ) t .beta. r ( .lamda. R ) - log I (
.lamda. R ) t .beta. r ( .lamda. IR ) log I ( .lamda. R ) t (
.beta. o ( .lamda. IR ) - .beta. r ( .lamda. IR ) ) - log I (
.lamda. IR ) t ( .beta. o ( .lamda. R ) - .beta. r ( .lamda. R ) )
##EQU00003##
Note in discrete time
log I ( .lamda. , t ) t log I ( .lamda. , t 2 ) - log I ( .lamda. ,
t 1 ) ##EQU00004##
Using log A-log B=log A/B,
[0043] log I ( .lamda. , t ) t log ( I ( t 2 , .lamda. ) I ( t 1 ,
.lamda. ) ) ##EQU00005##
So, (4) can be rewritten as
log I ( .lamda. R ) t log I ( .lamda. IR ) t log ( I ( t 1 ,
.lamda. R ) I ( t 2 , .lamda. R ) ) log ( I ( t 1 , .lamda. IR ) I
( t 2 , .lamda. IR ) ) = R ( 5 ) ##EQU00006##
where R represents the "ratio of ratios." Solving (4) for s using
(5) gives
s = .beta. r ( .lamda. R ) - R .beta. r ( .lamda. IR ) R ( .beta. o
( .lamda. IR ) - .beta. r ( .lamda. IR ) ) - .beta. o ( .lamda. R )
+ .beta. r ( .lamda. R ) . ##EQU00007##
From (5), R can be calculated using two points (e.g, PPG maximum
and minimum), or a family of points. One method using a family of
points uses a modified version of (5). Using the relationship
log I t = I / t I ( 6 ) ##EQU00008##
now (5) becomes
log I ( .lamda. R ) t log I ( .lamda. IR ) t I ( t 2 , .lamda. R )
- I ( t 1 , .lamda. R ) I ( t 1 , .lamda. R ) I ( t 2 , .lamda. IR
) - I ( t 1 , .lamda. IR ) I ( t 1 , .lamda. IR ) = [ I ( t 2 ,
.lamda. R ) - I ( t 1 , .lamda. R ) ] I ( t 1 , .lamda. IR ) [ I (
t 2 , .lamda. IR ) - I ( t 1 , .lamda. IR ) ] I ( t 1 , .lamda. R )
= R ( 7 ) ##EQU00009##
which defines a cluster of points whose slope of y versus x will
give R where
x(t)=[I(t.sub.2,.lamda..sub.IR)-I(t.sub.1,.lamda..sub.IR)]I(t.sub.1,.lam-
da..sub.R)
y(t)=[I(t.sub.2,.lamda..sub.R)-I(t.sub.1,.lamda..sub.R)]I(t.sub.1,.lamda-
..sub.IR) (8)
y(t)=Rx(t)
[0044] FIG. 1 is a perspective view of an embodiment of a pulse
oximetry system 10. System 10 may include a sensor 12 and a pulse
oximetry monitor 14. Sensor 12 may include an emitter 16 for
emitting light at two or more wavelengths into a patient's tissue.
A detector 18 may also be provided in sensor 12 for detecting the
light originally from emitter 16 that emanates from the patient's
tissue after passing through the tissue.
[0045] According to another embodiment and as will be described,
system 10 may include a plurality of sensors forming a sensor array
in lieu of single sensor 12. Each of the sensors of the sensor
array may be a complementary metal oxide semiconductor (CMOS)
sensor. Alternatively, each sensor of the array may be charged
coupled device (CCD) sensor. In another embodiment, the sensor
array may be made up of a combination of CMOS and CCD sensors. The
CCD sensor may comprise a photoactive region and a transmission
region for receiving and transmitting data whereas the CMOS sensor
may be made up of an integrated circuit having an array of pixel
sensors. Each pixel may have a photodetector and an active
amplifier.
[0046] According to an embodiment, emitter 16 and detector 18 may
be on opposite sides of a digit such as a finger or toe, in which
case the light that is emanating from the tissue has passed
completely through the digit. In an embodiment, emitter 16 and
detector 18 may be arranged so that light from emitter 16
penetrates the tissue and is reflected by the tissue into detector
18, such as a sensor designed to obtain pulse oximetry data from a
patient's forehead.
[0047] In an embodiment, the sensor or sensor array may be
connected to and draw its power from monitor 14 as shown. In
another embodiment, the sensor may be wirelessly connected to
monitor 14 and include its own battery or similar power supply (not
shown). Monitor 14 may be configured to calculate physiological
parameters based at least in part on data received from sensor 12
relating to light emission and detection. In an alternative
embodiment, the calculations may be performed on the monitoring
device itself and the result of the oximetry reading may be passed
to monitor 14. Further, monitor 14 may include a display 20
configured to display the physiological parameters or other
information about the system. In the embodiment shown, monitor 14
may also include a speaker 22 to provide an audible sound that may
be used in various other embodiments, such as for example, sounding
an audible alarm in the event that a patient's physiological
parameters are not within a predefined normal range.
[0048] In an embodiment, sensor 12, or the sensor array, may be
communicatively coupled to monitor 14 via a cable 24. However, in
other embodiments, a wireless transmission device (not shown) or
the like may be used instead of or in addition to cable 24.
[0049] In the illustrated embodiment, pulse oximetry system 10 may
also include a multi-parameter patient monitor 26. The monitor may
be cathode ray tube type, a flat panel display (as shown) such as a
liquid crystal display (LCD) or a plasma display, or any other type
of monitor now known or later developed. Multi-parameter patient
monitor 26 may be configured to calculate physiological parameters
and to provide a display 28 for information from monitor 14 and
from other medical monitoring devices or systems (not shown). For
example, multiparameter patient monitor 26 may be configured to
display an estimate of a patient's blood oxygen saturation
generated by pulse oximetry monitor 14 (referred to as an
"SpO.sub.2" measurement), pulse rate information from monitor 14
and blood pressure from a blood pressure monitor (not shown) on
display 28.
[0050] Monitor 14 may be communicatively coupled to multi-parameter
patient monitor 26 via a cable 32 or 34 that is coupled to a sensor
input port or a digital communications port, respectively and/or
may communicate wirelessly (not shown). In addition, monitor 14
and/or multi-parameter patient monitor 26 may be coupled to a
network to enable the sharing of information with servers or other
workstations (not shown). Monitor 14 may be powered by a battery
(not shown) or by a conventional power source such as a wall
outlet.
[0051] FIG. 2 is a block diagram of a pulse oximetry system, such
as pulse oximetry system 10 of FIG. 1, which may be coupled to a
patient 40 in accordance with an embodiment. Certain illustrative
components of sensor 12 and monitor 14 are illustrated in FIG. 2.
Sensor 12 may include emitter 16, detector 18, and encoder 42. In
the embodiment shown, emitter 16 may be configured to emit at least
two wavelengths of light (e.g., RED and IR) into a patient's tissue
40. Hence, emitter 16 may include a RED light emitting light source
such as RED light emitting diode (LED) 44 and an IR light emitting
light source such as IR LED 46 for emitting light into the
patient's tissue 40 at the wavelengths used to calculate the
patient's physiological parameters. In one embodiment, the RED
wavelength may be between about 600 nm and about 700 nm, and the IR
wavelength may he between about 800 nm and about 1000 nm. In
embodiments where a sensor array is used in place of single sensor,
each sensor may be configured to emit a single wavelength. For
example, a first sensor emits only a RED light while a second only
emits an IR light.
[0052] It will be understood that, as used herein, the term "light"
may refer to energy produced by radiative sources and may include
one or more of ultrasound, radio, microwave, millimeter wave,
infrared, visible, ultraviolet, gamma ray or X-ray electromagnetic
radiation. As used herein, light may also include any wavelength
within the radio, microwave, infrared, visible, ultraviolet, or
X-ray spectra, and that any suitable wavelength of electromagnetic
radiation may be appropriate for use with the present techniques.
Detector 18 may be chosen to be specifically sensitive to the
chosen targeted energy spectrum of the emitter 16.
[0053] In an embodiment, detector 18 may be configured to detect
the intensity of light at the RED and IR wavelengths.
Alternatively, each sensor in the array may be configured to detect
an intensity of a single wavelength. In operation, light may enter
detector 18 after passing through the patient's tissue 40. Detector
18 may convert the intensity of the received light into an
electrical signal. The light intensity is directly related to the
absorbance and/or reflectance of light in the tissue 40. That is,
when more light at a certain wavelength is absorbed or reflected,
less light of that wavelength is received from the tissue by the
detector 18. After converting the received light to an electrical
signal, detector 18 may send the signal to monitor 14, where
physiological parameters may be calculated based on the absorption
of the RED and IR wavelengths in the patient's tissue 40. An
example of a device configured to perform such calculations is the
Model N600x pulse oximeter available from Nellcor Puritan Bennett
LLC.
[0054] In an embodiment, encoder 42 may contain information about
sensor 12, such as what type of sensor it is (e.g., whether the
sensor is intended for placement on a forehead or digit) and the
wavelengths of light emitted by emitter 16. This information may be
used by monitor 14 to select appropriate algorithms, lookup tables
and/or calibration coefficients stored in monitor 14 for
calculating the patient's physiological parameters.
[0055] Encoder 42 may contain information specific to patient 40,
such as, for example, the patient's age, weight, and diagnosis.
This information may allow monitor 14 to determine, for example,
patient-specific threshold ranges in which the patient's
physiological parameter measurements should fall and to enable or
disable additional physiological parameter algorithms. Encoder 42
may, for instance, be a coded resistor which stores values
corresponding to the type of sensor 12 or the type of each sensor
in the sensor array, the wavelengths of light emitted by emitter 16
on each sensor of the sensor array, and/or the patient's
characteristics. In another embodiment, encoder 42 may include a
memory on which one or more of the following information may be
stored for communication to monitor 14: the type of the sensor 12;
the wavelengths of light emitted by emitter 16; the particular
wavelength each sensor in the sensor array is monitoring; a signal
threshold for each sensor in the sensor array; any other suitable
information; or any combination thereof.
[0056] In an embodiment, signals from detector 18 and encoder 42
may be transmitted to monitor 14. In the embodiment shown, monitor
14 may include a general-purpose microprocessor 48 connected to an
internal bus 50. Microprocessor 48 may be adapted to execute
software, which may include an operating system and one or more
applications, as part of performing the functions described herein.
Also connected to bus 50 may be a read-only memory (ROM) 52, a
random access memory (RAM) 54, user inputs 56, display 20, and
speaker 22.
[0057] RAM 54 and ROM 52 are illustrated by way of example, and not
limitation. Any suitable computer-readable media may be used in the
system for data storage. Computer-readable media are capable of
storing information that can be interpreted by microprocessor 48.
This information may be data or may take the form of
computer-executable instructions, such as software applications,
that cause the microprocessor to perform certain functions and/or
computer-implemented methods. Depending on the embodiment, such
computer-readable media may include computer storage media and
communication media. Computer storage media may include volatile
and non-volatile, removable and non-removable media implemented in
any method or technology for storage of information such as
computer-readable instructions, data structures, program modules or
other data. Computer storage media may include, but is not limited
to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state
memory technology, CD-ROM, DVD, or other optical storage, magnetic
cassettes, magnetic tape, magnetic disk storage or other magnetic
storage devices, or any other medium which can be used to store the
desired information and which can be accessed by components of the
system.
[0058] In the embodiment shown, a time processing unit (TPU) 58 may
provide timing control signals to a light drive circuitry 60, which
may control when emitter 16 is illuminated and multiplexed timing
for the RED LED 44 and the IR LED 46. TPU 58 may also control the
gating-in of signals from detector 18 through an amplifier 62 and a
switching circuit 64. These signals are sampled at the proper time,
depending upon which light source is illuminated. The received
signal from detector 18 may be passed through an amplifier 66, a
low pass filter 68, and an analog-to-digital converter 70. The
digital data may then be stored in a queued serial module (QSM) 72
(or buffer) for later downloading to RAM 54 as QSM 72 fills up. In
one embodiment, there may be multiple separate parallel paths
having amplifier 66, filter 68, and A/D converter 70 for multiple
light wavelengths or spectra received.
[0059] In an embodiment, microprocessor 48 may determine the
patient's physiological parameters, such as SpO.sub.2 and pulse
rate, using various algorithms and/or look-up tables based on the
value of the received signals and/or data corresponding to the
light received by detector 18. Signals corresponding to information
about patient 40, and particularly about the intensity of light
emanating from a patient's tissue over time, may be transmitted
from encoder 42 to a decoder 74. These signals may include, for
example, encoded information relating to patient characteristics.
Decoder 74 may translate these signals to enable the microprocessor
to determine the thresholds based on algorithms or look-up tables
stored in ROM 52. User inputs 56 may be used to enter information
about the patient, such as age, weight, height, diagnosis,
medications, treatments, and so forth. In an embodiment, display 20
may exhibit a list of values which may generally apply to the
patient, such as, for example, age ranges or medication families,
which the user may select using user inputs 56.
[0060] The optical signal through the tissue can be degraded by
noise and motion artifacts, among other sources. One source of
noise is ambient light that reaches the light detector. Another
source of noise is electromagnetic coupling from other electronic
instruments. Movement of the patient also introduces noise and
affects the signal. For example, the contact between the detector
and the skin, or the emitter and the skin, can be temporarily
disrupted when movement causes either to move away from the skin.
In addition, because blood is a fluid, it responds differently than
the surrounding tissue to inertial effects, thus resulting in
momentary changes in volume at the point to which the oximeter
probe is attached.
[0061] Motion artifact can degrade a pulse oximetry signal relied
upon by a physician, without the physician's awareness. This is
especially true if the monitoring of the patient is remote, the
motion is too small to be observed, or the doctor is watching the
instrument or other pails of the patient, and not the sensor site.
Processing pulse oximetry (i.e., PPG) signals may involve
operations that reduce the amount of noise present in the signals
or otherwise identify noise components in order to prevent them
from affecting measurements of physiological parameters derived
from the PPG signals.
[0062] It will be understood that the present disclosure is
applicable to any suitable signals and that PPG signals are used
merely for illustrative purposes. Those skilled in the art will
recognize that the present disclosure has wide applicability to
other signals including, but not limited to other biosignals (e.g.,
electrocardiogram, electroencephalogram, electrogastrogram,
electromyogram, heart rate signals, pathological sounds,
ultrasound, or any other suitable biosignal), dynamic signals,
non-destructive testing signals, condition monitoring signals,
fluid signals, geophysical signals, astronomical signals,
electrical signals, financial signals including financial indices,
sound and speech signals, chemical signals, meteorological signals
including climate signals, and/or any other suitable signal, and/or
any combination thereof.
[0063] In one embodiment, a PPG signal may be transformed using a
continuous wavelet transform. Information derived from the
transform of the PPG signal (i.e., in wavelet space) may be used to
provide measurements of one or more physiological parameters.
[0064] The continuous wavelet transform of a signal x(t) in
accordance with the present disclosure may be defined as
T ( a , b ) = 1 a .intg. - .infin. + .infin. x ( t ) .psi. * ( t -
b a ) t ( 9 ) ##EQU00010##
where .psi.*(t) is the complex conjugate of the wavelet function
.psi.(t), a is the dilation parameter of the wavelet and b is the
location parameter of the wavelet. The transform given by equation
(9) may be used to construct a representation of a signal on a
transform surface. The transform may be regarded as a time-scale
representation. Wavelets are composed of a range of frequencies,
one of which may be denoted as the characteristic frequency of the
wavelet, where the characteristic frequency associated with the
wavelet is inversely proportional to the scale a. One example of a
characteristic frequency is the dominant frequency. Each scale of a
particular wavelet may have a different characteristic frequency.
The underlying mathematical detail required for the implementation
within a time-scale can he found, for example, in Paul S. Addison,
The Illustrated Wavelet Transform Handbook (Taylor & Francis
Group 2002), which is hereby incorporated by reference herein in
its entirety.
[0065] The continuous wavelet transform decomposes a signal using
wavelets, which are generally highly localized in time. The
continuous wavelet transform may provide a higher resolution
relative to discrete transforms, thus providing the ability to
garner more information from signals than typical frequency
transforms such as Fourier transforms (or any other spectral
techniques) or discrete wavelet transforms. Continuous wavelet
transforms allow for the use of a range of wavelets with scales
spanning the scales of interest of a signal such that small scale
signal components correlate well with the smaller scale wavelets
and thus manifest at high energies at smaller scales in the
transform. Likewise, large scale signal components correlate well
with the larger scale wavelets and thus manifest at high energies
at larger scales in the transform. Thus, components at different
scales may be separated and extracted in the wavelet transform
domain. Moreover, the use of a continuous range of wavelets in
scale and time position allows for a higher resolution transform
than is possible relative to discrete techniques.
[0066] In addition, transforms and operations that convert a signal
or any other type of data into a spectral (i.e., frequency) domain
necessarily create a series of frequency transform values in a
two-dimensional coordinate system where the two dimensions may be
frequency and, for example, amplitude. For example, any type of
Fourier transform would generate such a two-dimensional spectrum.
In contrast, wavelet transforms, such as continuous wavelet
transforms, are required to be defined in a three-dimensional
coordinate system and generate a surface with dimensions of time,
scale and, for example, amplitude. Hence, operations performed in a
spectral domain cannot be performed in the wavelet domain; instead
the wavelet surface must be transformed into a spectrum (i.e., by
performing an inverse wavelet transform to convert the wavelet
surface into the time domain and then performing a spectral
transform from the time domain). Conversely, operations performed
in the wavelet domain cannot be performed in the spectral domain;
instead a spectrum must first be transformed into a wavelet surface
(i.e., by performing an inverse spectral transform to convert the
spectral domain into the time domain and then performing a wavelet
transform from the time domain). Nor does a cross-section of the
three-dimensional wavelet surface along, for example, a particular
point in time equate to a frequency spectrum upon which
spectral-based techniques may be used. At least because wavelet
space includes a time dimension, spectral techniques and wavelet
techniques are not interchangeable. It will be understood that
converting a system that relies on spectral domain processing to
one that relies on wavelet space processing would require
significant and fundamental modifications to the system in order to
accommodate the wavelet space processing (e.g., to derive a
representative energy value for a signal or part of a signal
requires integrating twice, across time and scale, in the wavelet
domain while, conversely, one integration across frequency is
required to derive a representative energy value from a spectral
domain). As a further example, to reconstruct a temporal signal
requires integrating twice, across time and scale, in the wavelet
domain while, conversely, one integration across frequency is
required to derive a temporal signal from a spectral domain. It is
well known in the art that, in addition to or as an alternative to
amplitude, parameters such as energy density, modulus, phase, among
others may all be generated using such transforms and that these
parameters have distinctly different contexts and meanings when
defined in a two-dimensional frequency coordinate system rather
than a three-dimensional wavelet coordinate system. For example,
the phase of a Fourier system is calculated with respect to a
single origin for all frequencies while the phase for a wavelet
system is unfolded into two dimensions with respect to a wavelet's
location (often in time) and scale.
[0067] The energy density function of the wavelet transform, the
scalogram, is defined as
S(a,b)=|T(a,b)|.sup.2 (10)
where `.parallel.` is the modulus operator. The scalogram may be
resealed for useful purposes. One common rescaling is defined
as
S R ( a , b ) = T ( a , b ) 2 a ( 11 ) ##EQU00011##
and is useful for defining ridges in wavelet space when, for
example, the Morlet wavelet is used. Ridges are defined as the
locus of points of local maxima in the plane. Any reasonable
definition of a ridge may be employed in the method. Also included
as a definition of a ridge herein are paths displaced from the
locus of the local maxima. A ridge associated with only the locus
of points of local maxima in the plane are labeled a "maxima
ridge".
[0068] For implementations requiring fast numerical computation,
the wavelet transform may be expressed as an approximation using
Fourier transforms. Pursuant to the convolution theorem, because
the wavelet transform is the cross-correlation of the signal with
the wavelet function, the wavelet transform may be approximated in
terms of an inverse FFT of the product of the Fourier transform of
the signal and the Fourier transform of the wavelet for each
required a scale and then multiplying the result by {square root
over (a)}.
[0069] In the discussion of the technology which follows herein,
the "scalogram" may be taken to include all suitable forms of
resealing including, but not limited to, the original unsealed
wavelet representation, linear resealing, any power of the modulus
of the wavelet transform, or any other suitable rescaling. In
addition, for purposes of clarity and conciseness, the term
"scalogram" shall be taken to mean the wavelet transform, T(a,b)
itself, or any part thereof For example, the real part of the
wavelet transform, the imaginary part of the wavelet transform, the
phase of the wavelet transform, any other suitable part of the
wavelet transform, or any combination thereof is intended to be
conveyed by the term "scalogram".
[0070] A scale, which may be interpreted as a representative
temporal period, may be converted to a characteristic frequency of
the wavelet function. The characteristic frequency associated with
a wavelet of arbitrary a scale is given by
f = f c a ( 12 ) ##EQU00012##
where f.sub.c, the characteristic frequency of the mother wavelet
(i.e., at a-1), becomes a scaling constant and f is the
representative or characteristic frequency for the wavelet at
arbitrary scale a.
[0071] Any suitable wavelet function may be used in connection with
the present disclosure. One of the most commonly used complex
wavelets, the Morlet wavelet, is defined as:
.psi.(t)=.pi..sup.-1/4(e.sup.i2.pi.f.sup.o.sup.'-e.sup.-(2.pi.f.sup.o.su-
p.).sup.2.sup./2)e.sup.-t.sup.2.sup./2 (13)
where f.sub.0 is the central frequency of the mother wavelet. The
second term in the parenthesis is known as the correction term, as
it corrects for the non-zero mean of the complex sinusoid within
the Gaussian window. In practice, it becomes negligible for values
of f.sub.0>>0 and can be ignored, in which case, the Morlet
wavelet can be written in a simpler form as
.psi. ( t ) = 1 .pi. 1 / 4 2.pi. f 0 t - t 2 / 2 ( 14 )
##EQU00013##
[0072] This wavelet is a complex wave within a scaled Gaussian
envelope. While both definitions of the Morlet wavelet are included
herein, the function of equation (14) is not strictly a wavelet as
it has a non-zero mean (i.e., the zero frequency term of its
corresponding energy spectrum is non-zero). However, it will be
recognized by those skilled in the art that equation (14) may be
used in practice with f.sub.0>>0 with minimal error and is
included (as well as other similar near wavelet functions) in the
definition of a wavelet herein. A more detailed overview of the
underlying wavelet theory, including the definition of a wavelet
function, can be found in the general literature. Discussed herein
is how wavelet transform features may be extracted from the wavelet
decomposition of signals. For example, wavelet decomposition of PPG
signals may be used to provide clinically useful information within
a medical device.
[0073] Pertinent repeating features in a signal give rise to a
time-scale band in wavelet space or a resealed wavelet space. For
example, the pulse component of a PPG signal produces a dominant
band in wavelet space at or around the pulse frequency. FIGS. 3(a)
and (b) show two views of an illustrative scalogram derived from a
PPG signal, according to an embodiment. The figures show an example
of the band caused by the pulse component in such a signal. The
pulse band is located between the dashed lines in the plot of FIG.
3(a). The band is formed from a series of dominant coalescing
features across the scalogram. This can be clearly seen as a raised
band across the transform surface in FIG. 3(b) located within the
region of scales indicated by the arrow in the plot (corresponding
to 60 beats per minute). The maxima of this band with respect to
scale is the ridge. The locus of the ridge is shown as a black
curve on top of the band in FIG. 3(b). By employing a suitable
resealing of the scalogram, such as that given in equation (11),
the ridges found in wavelet space may be related to the
instantaneous frequency of the signal. In this way, the pulse rate
may be obtained from the PPG signal. Instead of resealing the
scalogram, a suitable predefined relationship between the scale
obtained from the ridge on the wavelet surface and the actual pulse
rate may also be used to determine the pulse rate.
[0074] By mapping the time-scale coordinates of the pulse ridge
onto the wavelet phase information gained through the wavelet
transform, individual pulses may be captured. In this way, both
times between individual pulses and the timing of components within
each pulse may be monitored and used to detect heart beat
anomalies, measure arterial system compliance, or perform any other
suitable calculations or diagnostics. Alternative definitions of a
ridge may be employed. Alternative relationships between the ridge
and the pulse frequency of occurrence may be employed.
[0075] As discussed above, pertinent repeating features in the
signal give rise to a time-scale band in wavelet space or a
resealed wavelet space. For a periodic signal, this band remains at
a constant scale in the time-scale plane. For many real signals,
especially biological signals, the band may be non-stationary;
varying in scale, amplitude, or both over time. FIG. 3(c) shows an
illustrative schematic of a wavelet transform of a signal
containing two pertinent components leading to two bands in the
transform space, according to an embodiment. These bands are
labeled band A and band B on the three-dimensional schematic of the
wavelet surface. In an embodiment, the band ridge is defined as the
locus of the peak values of these bands with respect to scale. For
purposes of discussion, it may be assumed that band B contains the
signal information of interest. This will be referred to as the
"primary band". In addition, it may be assumed that the system from
which the signal originates, and from which the transform is
subsequently derived, exhibits some form of coupling between the
signal components in band A and band B. When noise or other
erroneous features are present in the signal with similar spectral
characteristics of the features of band B then the information
within band B can become ambiguous (i.e., obscured, fragmented or
missing). In this case, the ridge of band A may be followed in
wavelet space and extracted either as an amplitude signal or a
scale signal which will be referred to as the "ridge amplitude
perturbation" (RAP) signal and the "ridge scale perturbation" (RSP)
signal, respectively. The RAP and RSP signals may be extracted by
projecting the ridge onto the time-amplitude or time-scale planes,
respectively. The top plots of FIG. 3(d) show a schematic of the
RAP and RSP signals associated with ridge A in FIG. 3(c). Below
these RAP and RSP signals are schematics of a further wavelet
decomposition of these newly derived signals. This secondary
wavelet decomposition allows for information in the region of band
B in FIG. 3(c) to be made available as band C and band D. The
ridges of bands C and D may serve as instantaneous time-scale
characteristic measures of the signal components causing bands C
and D. This technique, which will be referred to herein as
secondary wavelet feature decoupling (SWFD), may allow information
concerning the nature of the signal components associated with the
underlying physical process causing the primary band B (FIG. 3(c))
to be extracted when band B itself is obscured in the presence of
noise or other erroneous signal features.
[0076] In some instances, an inverse continuous wavelet transform
may be desired, such as when modifications to a scalogram (or
modifications to the coefficients of a transformed signal) have
been made in order to, for example, remove artifacts. In one
embodiment, there is an inverse continuous wavelet transform which
allows the original signal to be recovered from its wavelet
transform by integrating over all scales and locations, a and
b:
x ( t ) = 1 C g .intg. - .infin. .infin. .intg. 0 .infin. T ( a , b
) 1 a .psi. ( t - b a ) a b a 2 ( 15 ) ##EQU00014##
which may also be written as:
x ( t ) = 1 C g .intg. - .infin. .infin. .intg. 0 .infin. T ( a , b
) .psi. a , b ( t ) a b a 2 ( 16 ) ##EQU00015##
where C.sub.g is a scalar value known as the admissibility
constant. It is wavelet type dependent and may be calculated
from:
C g = .intg. 0 .infin. .psi. ^ ( f ) 2 f f ( 17 ) ##EQU00016##
FIG. 3(e) is a flow chart of illustrative steps that may be taken
to perform an inverse continuous wavelet transform in accordance
with the above discussion. An approximation to the inverse
transform may be made by considering equation (15) to be a series
of convolutions across scales. It shall be understood that there is
no complex conjugate here, unlike for the cross correlations of the
forward transform. As well as integrating over all of a and b for
each time t, this equation may also take advantage of the
convolution theorem which allows the inverse wavelet transform to
be executed using a series of multiplications. FIG. 3(f) is a flow
chart of illustrative steps that may be taken to perform an
approximation of an inverse continuous wavelet transform. It will
be understood that any other suitable technique for performing an
inverse continuous wavelet transform may be used in accordance with
the present disclosure.
[0077] FIG. 4 is an illustrative continuous wavelet processing
system in accordance with an embodiment. In an embodiment, input
signal generator 410 generates an input signal 416. As illustrated,
input signal generator 410 may include oximeter 420 coupled to
sensor 418, which may provide as input signal 416, a PPG signal. It
will be understood that input signal generator 410 may include any
suitable signal source, signal generating data, signal generating
equipment, or any combination thereof to produce signal 416. Signal
416 may be any suitable signal or signals, such as, for example,
biosignals (e.g., electrocardiogram, electroencephalogram,
electrogastrogram, electromyogram, heart rate signals, pathological
sounds, ultrasound, or any other suitable biosignal), dynamic
signals, non-destructive testing signals, condition monitoring
signals, fluid signals, geophysical signals, astronomical signals,
electrical signals, financial signals including financial indices,
sound and speech signals, chemical signals, meteorological signals
including climate signals, and/or any other suitable signal, and/or
any combination thereof.
[0078] In an embodiment, signal 416 may be coupled to processor
412. Processor 412 may be any suitable software, firmware) and/or
hardware, and/or combinations thereof for processing signal 416.
For example, processor 412 may include one or more hardware
processors (e.g., integrated circuits), one or more software
modules, computer-readable media such as memory, firmware, or any
combination thereof. Processor 412 may, for example, be a computer
or may be one or more chips (i.e., integrated circuits). Processor
412 may perform the calculations associated with the continuous
wavelet transforms of the present disclosure as well as the
calculations associated with any suitable interrogations of the
transforms. Processor 412 may perform any suitable signal
processing of signal 416 to filter signal 416, such as any suitable
band-pass filtering, adaptive filtering, closed-loop filtering,
and/or any other suitable filtering, and/or any combination
thereof.
[0079] Processor 412 may be coupled to one or more memory devices
(not shown) or incorporate one or more memory devices such as any
suitable volatile memory device (e.g., RAM, registers, etc.),
non-volatile memory device (e.g., ROM, EPROM, magnetic storage
device, optical storage device, flash memory, etc.), or both. The
memory may be used by processor 412 to, for example, store data
corresponding to a continuous wavelet transform of input signal
416, such as data representing a scalogram. In one embodiment, data
representing a scalogram may be stored in RAM or memory internal to
processor 412 as any suitable three-dimensional data structure such
as a three-dimensional array that represents the scalogram as
energy levels in a time-scale plane. Any other suitable data
structure may be used to store data representing a scalogram.
[0080] Processor 412 may be coupled to output 414. Output 414 may
be any suitable output device such as, for example, one or more
medical devices (e.g., a medical monitor that displays various
physiological parameters, a medical alarm, or any other suitable
medical device that either displays physiological parameters or
uses the output of processor 412 as an input), one or more display
devices (e.g., monitor, PDA, mobile phone, any other suitable
display device, or any combination thereof), one or more audio
devices, one or more memory devices (e.g., hard disk drive, flash
memory, RAM, optical disk, any other suitable memory device, or any
combination thereof, one or more printing devices, any other
suitable output device, or any combination thereof.
[0081] It will be understood that system 400 may be incorporated
into system 10 (FIGS. 1 and 2) in which, for example, input signal
generator 410 may be implemented as parts of sensor 12 and monitor
14 and processor 412 may be implemented as part of monitor 14.
[0082] FIG. 5 shows an illustrative plot of a PPG signal 510 taken
during a period of decreasing signal quality in accordance with an
embodiment. Plot 500 displays time on the x-axis and light
intensity on the y-axis. The y-axis may represent the light
intensity detected by detector 18 (FIG. 1) that emanates from the
tissue of patient 40 (FIG. 1). Larger values on the y-axis indicate
larger light intensity measurements than smaller values on the
y-axis (for example, light intensity value 502 represents a larger
light intensity than light intensity value 504). PPG signal 510 may
be obtained, for example, from sensor 12 (FIG. 1) or from averaging
or otherwise combining a plurality of signals derived from a
suitable sensor array, as discussed in relation to FIG. 1. Plot 500
may be displayed using any suitable display device such as, for
example, monitor 20 (FIG. 1), display 28 (FIG. 1), a PDA, a mobile
phone, or any other suitable display device. Additionally, plot 500
may be displayed on multiple display devices, or it may not be
displayed on any display devices.
[0083] A period of decreasing signal quality is a period in which
the "quality" of the PPG signal 510 decreases in some way. The
quality of PPG signal 510 may be determined using a system such as
system 10 (FIGS. 1 and 2) and/or system 400 (FIG. 4) to perform a
suitable analysis of the signal. For example, the quality of PPG
signal 510 may be characterized by analyzing the energy of the
signal or by calculating the signal-to-noise level of the signal.
These characteristics may be calculated, for example, from a
suitable scalogram of PPG signal 510, and in particular, may be
calculated using one or more portions of such a scalogram.
[0084] A period of decreasing signal quality may indicate that the
intended target stimulus (e.g., patient's 40 (FIG. 2) fingertip,
toe, forehead, earlobe, or foot) is not being adequately measured
by, for example, pulse oximetry system 10 (FIGS. 1 and 2). Possible
causes of a period of decreasing signal quality may include: sensor
20 (FIG. 1) being slowly dislodged from patient 40 (FIG. 2), sensor
20 (FIG. 1) or any constituent component of the sensor 20 (FIG. 1)
being damaged or otherwise malfunctioning, and/or a connecting
cable (e.g., cable 24, 32, or 34 of FIG. 1) being gradually removed
or otherwise malfunctioning. For example, PPG signal 510 was
generated during an experiment in which the pulse oximeter probe
was gradually loosened from the finger of patient 40 (FIG. 2). A
related example of a PPG signal for which there is a period of
decreased signal quality will be shown in FIG. 9.
[0085] Plot 500 may contain several characteristics that may be
used either individually or in combination to identify a period of
decreasing signal quality. Plot 500 is comprised of time periods
520, 530, and 540. Time period 520 may correspond to a time period
before a period of decreasing signal quality or it may correspond
to a period in which a decreasing signal quality is largely
imperceptible in PPG signal 510. In time period 520, PPG signal 510
has a relatively small light intensity amplitude and exhibits
relatively large oscillations in the light intensity amplitude. A
small light intensity amplitude may mean that a reduced amount of
light is measured at detector 18 (FIG. 1), which may indicate that
the target stimulus is being properly measured. Similarly, large
oscillations in the amplitude may indicate the presence of strong
pulse signal from patient 40 (FIG. 2), which may also indicate that
the target stimulus is being properly measured.
[0086] In time period 530, PPG signal 510 exhibits a generally
increasing light intensity amplitude and smaller oscillations in
the light intensity amplitude compared to those exhibited in time
period 520. These features may indicate that the signal quality of
PPG signal 510 is decreasing. Such a decreasing trend in the signal
quality of PPG signal 510 may be caused by any of a variety of
factors, such as those discussed above. In addition to the these
general trends, PPG signal 510 may exhibit scattered spurious
effects that are characterized by rapid, and possibly temporary,
changes in the light intensity amplitude. Examples include
amplitude increase 550 and/or amplitude fluctuation 560. Amplitude
increase 550 may correspond to, for example, a rapid and partial
loosening of the pulse oximeter probe on patient 40 which causes
more light to reach detector 18 (FIG. 1) from emitter 16 (FIG. 1).
Amplitude fluctuation 560 may correspond to, for example, a
temporary tightening and then re-loosening of the pulse oximeter
probe on the target stimulus on patient 40 (FIG. 2), which
temporarily decreases the amount of light received by detector 18
(FIG. 1).
[0087] Time period 540 may correspond to, for example, the case
where the pulse oximeter probe is nearly or completely removed from
the target stimulus on patent 40 (FIG. 2). In time period 540, PPG
signal 510 reaches an approximately constant light intensity
amplitude value 506. Also, PPG signal 510 exhibits only small
oscillations in light intensity amplitude during this period. These
characteristics may indicate that the intended target stimulus on
patient 40 (FIG. 2) is no longer being measured to a significant
degree.
[0088] As mentioned above, plot 500 was generated during an
experiment in which the pulse oximeter probe was gradually loosened
from the finger of patient 40 (FIG. 2) over time. However, as
emphasized above, plot 500 is merely illustrative of a general PPG
signal that may be obtained from, for example, pulse oximetry
system 10 (FIG. 1) or system 400 (FIG. 4). Further, and as
emphasized above, the target stimulus need not correspond to a
patient finger, as many other target stimuli would produce a plot
similar to plot 500. The actual rate of light intensity amplitude
decrease may be faster or more gradual than that shown in time
period 530, and the nature and number of energy increases and
energy fluctuations in PPG signal 510 may be unpredictable and
variable during time period 520.
[0089] FIG. 6 shows an illustrative scalogram 600 of a wavelet
transform derived from a PPG signal such as PPG signal 510 (FIG. 5)
during a period of decreasing signal quality in accordance with an
embodiment. In scalogram 600, the x-axis of denotes time and the
y-axis denotes scale. In scalogram 600, "hotter" colors (e.g., hues
of red, orange and yellow) correspond to larger energy values,
while "cooler" colors (e.g., hues of blue and green) correspond to
smaller energy values. Dark red, which is the color of region 680,
represents the largest energy value in scalogram 600, whereas dark
blue, which is the color of region 690, represents the smallest
energy values in scalogram 600. The regions in the lower left and
lower right corners of the plot may contain energy values that
reflect edge effects of the wavelet transform. These regions may be
of a very high energy, and in scalogram 600 these regions have been
replaced by energy values corresponding to the lowest energy values
present in scalogram 600. This has been done so that energy values
in these regions do not adversely influence the calculation of
color scale used to generate scalogram 600. These regions may be
ignored in the analysis of the scalogram.
[0090] Scalogram 600 comprises at least three distinct scale bands
(i.e., ranges of scale values): pulse band 610, mains hum band 620,
and noise band 630. Pulse band 610 may contain an energy structure
and energy values that reflect the pulse component of PPG signal
510 (FIG. 5). When a pulse component is present in PPG signal 510
(FIG. 5) (e.g, when an accurate measurement of a target stimulus is
made), pulse band 610 may be characterized by moderate to high
energy values within the band and areas of lower energy at
surrounding scale values. When a pulse component is not present in
PPG signal 510 (FIG. 5) (e.g., when an accurate measurement of the
target stimulus is not made), it may be expected that pulse band
610 will contain less energy. Mains hum band 620 may contain an
energy structure and energy values that reflect the electrical or
power line hum that is often characteristic of electric devices.
The scales range that defines the mains hum band 620 may depend on
characteristics of the alternating current supply (e.g., as used by
pulse oximetry system 10 (FIG. 1)). When such a mains hum component
is present in PPG signal 510 (FIG. 5), mains hum band 610 may be
characterized by regular and rapidly oscillating streaks of low to
moderate energy and areas of lower energy at surrounding scales.
Noise band 630 may contain an energy structure and energy values
that reflect general types of noise that may be present in PPG
signal 510 (FIG. 5). For example, noise band 630 may be include the
effects of thermal noise, shot noise, flicker noise, burst noise,
and/or electrical noise caused by light pollution. Noise band 610
may be characterized as having less energy structure than either
pulse band 610 or mains hum band 620, and as containing
low-to-moderate energy values.
[0091] In scalogram 600, time periods 640, 650, and 660 correspond
to the time periods 520, 530, and 540, respectively, as discussed
in FIG. 5. Therefore, time period 640 corresponds to the time
period for which the effect of the target stimulus is nearly or
fully captured in the measurement of PPG signal 510 (FIG. 5). Time
period 630 corresponds to a time period of decreasing signal
quality, and time period 660 corresponds to the time period after
which the effect of the target stimulus is largely or completely
uncaptured in measurement of PPG signal 510 (FIG. 5). Within pulse
band 610, energy values are seen to be moderate in time period 640,
decreasing from moderate to low in time period 650, and low in time
period 660. This may reflect the diminishing presence of a
measurable pulse component signal in PPG signal 510 (FIG. 5) during
time period 650. In contrast, the energy in the mains hum band 620
remains approximately constant throughout time periods 640, 650,
and 660. This is because the mains hum noise is generated by
electrical circuitry and may not depend on the signal that is
measured (e.g., by detector 18 (FIG. 1)). The energy in noise band
630 decreases from moderate and low energy values in time period
640 to very low energy values in time period 660. This is because
certain components of PPG signal 510 (FIG. 5) are contained within
the scales comprising the noise band 630. Therefore, as the quality
of the PPG signal 510 (FIG. 5) decreases, these components are not
measured by, for example, detector 18 (FIG. 1), which results in
less energy being detected in noise band 630.
[0092] Broadscale high-energy cone 670 is a sporadic effect caused
by energy fluctuation 560 (FIG. 5) and is characterized by a
cone-shaped region of high-energy that has a width that decreases
as the scale value increases. The location and number of broadscale
high-energy cones is in general variable and may be unpredictable
in advance. However, the presence of one or more broadscale
high-energy cones in a time period such as time period 650 may be
indicative a signal quality decrease in that time period.
[0093] FIG. 7 shows illustrative plots of an energy measure versus
time derived from scalogram 600 (FIG. 6). Each plot in FIG. 7 has
been calculated by taking the tenth-percentile of the energy
density values in scalogram 600 (FIG. 6) over a 10-second long
time-window that includes a certain range of scale values. For
example, plot 710 has been generated by selecting scale values in
pulse band 610 RIG. 6), plot 720 has been generated by selecting
scale values in the mains hum band 620 (FIG. 6), and plot 730 has
been generated by selecting scale values in the noise band 630
(FIG. 6). The energy measure plotted in FIG. 7 has been plotted
using a logarithmic scale on the y-axis. It should be noted that
plot 710 has the largest amplitudes followed by plot 720 and then
plot 730. This is expected because, as discussed in relation to
scalogram 600 (FIG. 6), pulse band 610 (FIG. 6) has the largest
energy, followed by mains hum band 620 (FIG. 6) and noise band 630
(FIG. 6). Plot 710 exhibits a decrease in the energy measure in the
pulse band versus time (e.g., plot 710 has a value of approximately
4 at time 10, and decreases to a value of approximately 2.25 at
time 180) and plot 720 shows a relatively constant energy value in
time (e.g., plot 720 has a value of approximately 2.1 at time 10,
and a value of approximately 1.9 at time 180). These results are
expected because, as discussed in relation to scalogram 600 (FIG.
6), pulse band 610 (FIG. 6) decreases in energy versus time, while
mains hum band 620 (FIG. 6) has an approximately constant energy
versus time. Plot 730 shows a small to moderate energy decrease in
energy versus time (e.g., plot 730 has a value of approximately 0
at time 10, and a value of approximately -0.6 at time 180). In
general, the level of decrease in plot 730 depends, at least in
part, on the percentile threshold chosen in to generate plot 730.
The energy used above is merely illustrative. Alternative energy
measures (i.e., other than taking the tenth-percentile value in the
window) can be employed. For example, summing all of the values of
the lowest tenth-percentile may also be used.
[0094] In one embodiment, plots 710, 720, and/or 730 may be
monitored and/or combined, and used to determine when and if a
signal quality decrease has occurred or if one may occur. For
example, plots 710, 720, and 730 may be parameterized through, for
example, curve fitting using a linear straight line fit or a
nonlinear curve fit. Alternatively, or in combination, plots 710,
720, and 730 may be combined through any suitable operation that,
for example, weighs the values present in each plot or in some
subset of plots. This weighted data may be compared to a threshold
to determine if a signal quality decrease has occurred. Further,
many changes to the parameters and features used to generate plots
such as the ones illustrated in FIG. 7 may be made in accordance
with an embodiment. For example, in choosing regions of the
scalogram 600 (FIG. 6) over which to sum the energy density, the
length of the time-window may be shortened or lengthened and may be
chosen to include non-continuous segments; the selection of scale
values used to compute each plot may be altered (e.g., to use more
or fewer scale values); and/or a different threshold percentile
(i.e., other than 10-percent) may be chosen in computing plot 730.
Alternatively, instead of computing the tenth-percentile of energy
density, plots may be generated by another energy measure. For
example, plots may be generated in which the energy measure
computes the lowest fifth-percentile, twentieth-percentile, or any
other suitable percentile of energy values. Further, other
properties of the wavelet transform can be use to generate plots
other than or in addition to those of 710, 720, and 730. For
example, the real and/or imaginary components, and various powers
of the modulus and phase may be used.
[0095] FIG. 8 shows illustrative plots of signal-to-noise levels
versus time derived from FIG. 7. Plots 810 and 820 represent an
illustrative technique for weighing the information in plots 710,
720, and 730 from FIG. 7 in accordance with an embodiment. Plot 810
is plot of the ratio of pulse band 610 (FIG. 6) energy to the noise
band 620 (FIG. 6) energy and is obtained by dividing, at each point
in time, the value of plot 710 (FIG. 7) by the value of plot 730
(FIG. 7). Plot 820 is a plot of the ratio of the pulse band 610
(FIG. 6) energy to the mains hum band 620 (FIG. 6) energy and is
obtained by dividing, at each point in time, the value of plot 710
(FIG. 7) by the value of plot 720 (FIG. 7). Signal-to-noise level
plots such as 810 and 820 may be useful at least because they
provide a measure of how the signal energy changes relative to a
background noise level. For example, a large decrease in the signal
energy might be tolerable, and not indicative of a signal quality
decrease, if accompanied by a correspondingly large decrease in the
noise level. In one embodiment, plots 810 and/or 820 may be
combined and used to determine when a signal quality decrease
occurs. For example, plots 810 and 820 may be combined through any
suitable operation that weighs the values present in each plot to
generate a new plot, which may be used to determine if a signal
quality decrease has occurred. Plots 810 and 820 each have a signal
decrease of roughly 1.5 from time 10 to time 180. In one
embodiment, a signal quality decrease may be detected by comparing
the average decrease from time 10 to time 180 between plots 810 and
820 to a threshold. If the average decrease exceeds this threshold
value, then a signal quality decrease may be said to occur. The
threshold may be calculated using any suitable technique In one
embodiment, such a threshold may be determined using statistical
techniques such Neyman-Pearson hypothesis testing or the
maximum-likelihood detection. Alternatively, such a threshold may
be determined using historical data on signal-to-noise levels
calculated before and during a signal quality decrease event.
Alternatively or in combination, plots 810 and 820 may be
parameterized through, for example, curve fitting using a linear
straight line fit or a nonlinear curve fit before being used to
determine whether a signal quality decrease has occurred.
[0096] FIG. 9 shows an illustrative plot of a PPG signal 910 taken
prior to and during a period which includes a motion artifact in
accordance an embodiment. As will be explained below, the
occurrence of a motion artifact may result in a signal quality
decrease in PPG signal 910. The definitions and meanings of the
x-axis and y-axis in plot 900 are the same as for plot 500 (FIG.
5).
[0097] Plot 900 includes time periods 920 and 930. Time period 920
corresponds to a time period before a time period in which a motion
artifact is present in PPG signal 910. Starting at approximately
time 930, a significant motion artifact is measured in PPG signal
910. Such a motion artifact may be caused by, for example,
voluntary or involuntary respiration, eye movements, swallowing,
yawning, cardiac motion, and/or general body movement of patient
40. At approximately time 930, a motion artifact occurs in PPG
signal 910. Time period 940 corresponds to a period in which a
motion artifact remains present and measured by, for example,
sensor 12. The presence of the motion artifact leads to a distinct
change in PPG signal 910 during time period 940. The PPG signal
exhibits larger and less smooth oscillations in light intensity
amplitude during time period 940 than during time period 920.
Further, the light intensity amplitudes exhibited in time period
940 are generally smaller than those exhibited in time period 920.
These features may be used either singly or in combination to
identify a period of decreased signal quality caused by a
significant motion artifact or another related phenomena in PPG
signal 910.
[0098] FIG. 10 shows an illustrative scalogram derived from the PPG
signal 910 (FIG. 9) in accordance with an embodiment. The axes
values and the meanings of the colors in scalogram 1000 are the
same as for scalogram 600 (FIG. 6). In scalogram 1000, region 1010
(having dark blue and light blue colors) and the lower-left and
lower-right portions of the plot (each having a dark blue color)
are the regions of the smallest energy, whereas scale band 1020 and
region 1030 (having mostly dark red colors) are the regions of the
largest energy, and region 1040 (having mostly yellow and orange
colors) is a region of moderate energy. In scalogram 1000, time
periods 1050 and 1060 correspond to the time periods 920 and 940,
respectively, discussed above with respect to FIG. 9. Therefore,
time period 1050 corresponds to a time period before the appearance
and measurement of a motion artifact, and time period 1060
corresponds to period in which the motion artifact is present and
measured. The motion artifact is first measured at time 1070 which
corresponds to time 930 (FIG. 9). Scalogram 1000 exhibits different
characteristics in time period 1050 than in time period 1060. The
energy structure of scalogram 1000 is more random and the energy
amplitudes of scalogram 1000 are larger in time period 1050 than in
time period 1060. For example, the range of scales below scale band
1020 contain mostly low energy values in time period 1050 but
mostly moderate and high energy values in time 1060. The range of
scales immediately above scale band 1020 contain structured and
repeated energy characteristics in time period 1050 but relatively
less well-defined energy characteristics in time period 1060. These
features can be used to detect the presence of a signal quality
decrease due to a motion artifact or other related phenomena.
[0099] FIG. 11 is a flow chart 1100 of illustrative steps for
determining and responding to a decrease in signal quality in
accordance with an embodiment. Flow chart 1100 may begin at step
1102. At step 1104, a portion of a suitable signal may be obtained
using, for example, pulse oximetry system 10 (FIGS. 1 and 2) or
system 400 (FIG. 4). The signal may be obtained from a target
stimulus provided by patient 40 (FIG. 2). The signal obtained may
be a PPG signal. At step 1106, the wavelet transform of the signal
may be obtained. Such a wavelet transform may be obtained, for
example, by system 10 (FIGS. 1 and 2) or system 400 (FIG. 4). At
step 1108, the scalogram of the wavelet transform may be generated
or otherwise obtained using, for example a processor. For example,
the scalogram of the wavelet transform may be generated or obtained
using a processor such as processor 412 (FIG. 4) or microprocessor
48 (FIG. 2).
[0100] In addition to the scalogram, other parts of the wavelet
transform may be inspected to determine whether a signal quality
decrease event has occurred. For example, the transform modulus,
phase, real, and/or imaginary parts may be generated at step 1108
in place of or in addition to the scalogram. Each of these features
may then be used, either individually or in combination, in the
subsequent steps of flow chart 1100. For example, the transform
modulus or real, and/or imaginary values corresponding to the
wavelet transform may be summed or otherwise manipulated to
generate plots that can be used along with, or instead of, the
plots shown in FIG. 7 and FIG. 8 to determine whether a signal
quality decrease event has occurred. Alternatively or in addition
to the method describe above, the phase of a wavelet transform may
be analyzed across a range of scale values. The stability of phase
values may indicate a relative value of the signal quality, and may
be used to determine whether a signal quality decrease event has
occurred.
[0101] Referring back to FIG. 11, at step 1110, the scalogram
obtained in step 1108 may be quantized. Quantization refers to a
process of truncating a continuous or (high-precision) digital
signal value to a nearest reference value. The number of reference
values may be significantly smaller than the number of values
present in the signal prior to quantization. Quantization may be
beneficial at least for decreasing the complexity of hardware and
software resources required to process and store the scalogram
obtained in step 1108, as well as to further aid in the detection
and analysis of features of the scalogram in subsequent steps 1112
and 1114. Quantization may provide these benefits with only a small
or imperceptible degradation in the quality of the quantized signal
relative to the quality of the signal prior to quantization. Any
suitable quantization scheme may be used in step 1110. For example,
quantization of the scalogram obtained in step 1108 may be
performed using one, two, or multiple thresholds, whereby the
quantized scalogram is obtained by rounding energy values of the
original scalogram to the nearest threshold value. The threshold
value may be calculated using any suitable technique. In one
embodiment, such a threshold may be determined using statistical
techniques such Neyman-Pearson hypothesis testing or the
maximum-likelihood detection. Alternatively, such a threshold may
be determined using historical data on signal-to-noise levels
calculated before and during a signal quality decrease event. In
addition, the number and value of quantization levels may be chosen
based on the dynamic range of scalogram obtained in step 1108, the
computational resources available, or based on a combination of
these and any other suitable factors. Each threshold may be a
variable quantity that varies with, for example, the time or scale
value. It will be understood that step 1110, as well all other
steps of flow chart 1100, is optional and that quantization of the
scalogram need not be performed.
[0102] Referring back to FIG. 11, at step 1112, one or more
characteristics of the scalogram obtained in step 1108 or 1110 may
be determined using a processor. One or more of the characteristics
that is determined may be chosen to be beneficial in determining if
a signal quality decrease has occurred. For example,
characteristics that may be determined include the energy and
structure of the scalogram in pulse band 610 (FIG. 6), mains hum
band 620 (FIG. 6), and/or noise band 630 (FIG. 6), and the
signal-to-noise levels in various regions of scalogram 600 (FIG.
6). In one embodiment, this information may be calculated one or
more times using different time-window sizes. The number and type
of time-window sizes that are used may depend on the anticipated
rate of a possible signal quality decrease, the available
computational resources (e.g., the amount of ROM 52 (FIG. 2) and/or
RAM 54 (FIG. 2) and the speed of processor 412 (FIG. 4) and/or
microprocessor 48 (FIG. 2)), as well as on possible input derived
from user inputs 56 (FIG. 2).
[0103] Referring back to FIG. 11, at step 1114, the characteristics
determined in step 1112 may be analyzed. Analyzing the
characteristics may generally involve parsing, combining, and/or
weighing individual results obtained in the current and possible
previous iterations of step 1114 so that a single, overall decision
may be made as to whether a signal quality decrease has occurred.
Step 1114 may incorporate the use of past scalogram data that has
been obtained in previous iterations of process 1100 to determine
current signal quality values and also trends in the signal quality
values. For example, a signal quality value may be represented by a
number from 0 to 100, where a larger number indicates a higher
quality signal, and a trend may be represented by a number
representing a rate increase or decrease in the signal quality.
Past scalogram data may be stored in, for example, ROM 52 (FIG. 2)
and/or RAM 54 (FIG. 2). Step 1114 may also involve the
parameterization and/or curve fitting of data obtained in step 1112
using, for example, linear least-squares fitting of data or any
other suitable interpolation technique. Such parameterization
and/or curve fitting may be performed, for example, by processor
412 (FIG. 4) or microprocessor 48 (FIG. 2), and may additionally
depend on parameters entered by a user through user inputs 56 (FIG.
2). In step 1114, multiple signal quality values and trend data may
be combined to produce a simple form of data that may be used to
make a single overall decision as to the possible presence of a
signal quality decrease in a PPG signal such as PPG signal 510
(FIG. 5) or PPG signal 910 (FIG. 9). Any suitable parsing,
combining, and/or weighing strategy may be used. For example,
maximum-likelihood techniques may be used to combine data when the
prior probability of a signal decrease event is known, and
Neyman-Pearson combining techniques may be used when the prior
probability of a signal quality decrease event is unknown. In
addition, simply majority-vote decision rules may be used to
determine if a signal quality decrease has occurred.
[0104] Referring back to FIG. 11, at step 1116, a decision may be
made as to whether a decrease in signal quality has occurred. Such
a decision may be made based on the output of step 1114. If it is
determined that a decrease in signal quality has occurred, a
response may be initiated in step 1118. A response may include many
features singly or in combination. For example, possible features
may include generating an audible alert or alarm that is emitted,
for example, using speaker 22 (FIG. 2) as well as possibly through
other audio devices, generating an on-screen message, for example,
on display 20 (FIG. 1) or display 28 (FIG. 1), generating a pager
message, a text message, or a telephone call, for example, using a
wireless connection embedded or attached to a system such as system
10 (FIG. 1), activating a secondary or backup sensor or sensor
array, for example, connected through a wire or wirelessly to
monitor 14 (FIG. 1), or regulating the automatic administration
medicine, for example, which is controlled in part or fully through
a system such as system 10 (FIG. 1). If it is determined that a
signal quality decrease has not occurred, then process 1100 returns
to step 1104 and the "next" portion of the signal is obtained. The
next portion of the signal may start where the previously read
signal ended, overlap with the previously read signal, or be
located at some distance in the future from the previously read
signal. In any of these or in other scenarios, the choice of the
signal region to be selected may be influenced by the data
determined in step 1112 or analyzed in step 1114.
[0105] FIG. 12 shows a flow chart of illustrative steps for
performing step 1112 of FIG. 11 (i.e., for determining a set of
characteristics corresponding to a scalogram) in accordance with an
embodiment. At step 1210, one or more time-windows may be
determined and used to calculate the energy and other
characteristics of the scalogram determined in step 1108 or step
1110. For example, and as described previously in relation to FIG.
7, the energy values and structural characteristics in the pulse
band 610 (FIG. 6), mains hum band 620 (FIG. 6), and noise band 630
(FIG. 6) may be calculated. A separate time-window may be
determined for each of these scale bands as well as other scale
bands, and separate time-windows may be determined to measure
energy values and energy structural characteristics. Time-windows
may be of the same or different lengths, and each time-window may
be comprised of either continuous or discontinuous ranges of
time.
[0106] Referring back to FIG. 12, at step 1220, the energy values
and energy structure in pulse band 610 (FIG. 6) may be calculated,
for example, using one or more time-windows determined in step
1210. Energy values may be calculated by averaging the energy
densities of a scalogram such as scalogram 600 (FIG. 6) within a
given time-window as described in relation to FIG. 7, or through
any other suitable technique. For example, energy may be calculated
by averaging the energy density only over those energy values below
a certain percentile threshold in pulse band 610 (FIG. 6), or by
averaging only the minimum or maximum values at each time instant.
Alternatively, the energy structure in pulse band 610 (FIG. 6) may
be calculated by recording the presence of features within a given
time-window such as the number of and frequency of repeated
patterns, the presence of high energy regions followed by low
energy regions, and/or any other suitable characteristics.
[0107] Referring back to FIG. 12, at step 1230, the energy values
and energy structure in mains hum band 620 (FIG. 6) may be
calculated, for example, using one or more time-windows determined
in step 1210. Energy values may be calculated by averaging the
energy densities of a scalogram such as scalogram 600 (FIG. 6)
within a given time-window as described in relation to FIG. 7, or
through any other suitable technique. For example, energy may be
calculated by averaging the energy density only over energy values
below a certain percentile threshold in mains hum band 620 (FIG.
6), or by averaging only the minimum or maximum values at each time
instant. Alternatively, the energy structure in mains hum band 620
(FIG. 6) may be calculated by recording the presence of features
within a given time-window such as the number of and frequency of
repeated patterns, the presence of high energy regions followed by
low energy regions, and/or any other suitable characteristics.
[0108] Referring back to FIG. 12, at step 1240, the energy values
and energy structure in noise band 630 (FIG. 6) may be calculated,
for example, using one or more time-windows determined in step
1210. Energy values may be calculated by averaging the energy
densities of a scalogram such as scalogram 600 (FIG. 6) within a
given time-window as described in relation to FIG. 7, and/or
through any other suitable scheme. For example, energy may be
calculated by averaging the energy density only over energy values
below a certain percentile threshold in noise band 630 (FIG. 6), or
by averaging only the minimum or maximum values at each time
instant. Alternatively, the energy structure in noise band 630
(FIG. 6), may be calculated by recording the presence of features
within a given time-window such as the number of and frequency of
repeated patterns, the presence of high energy regions followed by
low energy regions, and/or any other suitable characteristics.
[0109] Referring back to FIG. 12, at step 1250, signal-to-noise
levels may be calculated corresponding to the characteristics
determined in steps 1220, 1230, and 1240 above. One or more
signal-to-noise levels may be calculated as described in relation
to FIG. 8 or through any other suitable scheme. For example, the
signal-to-noise level between the pulse band 610 (FIG. 6) and mains
hum band 620 (FIG. 6) may be calculated by dividing, at each time
point, the energy value obtained in step 1220 by the energy value
obtained in step 1230. Alternatively, the signal-to-noise level
between the pulse band 610 (FIG. 6) and noise band 630 (FIG. 6) may
be calculated by dividing, at each time point, the energy value
obtained in step 1220 by the energy value obtained in step 1240. At
step 1260, the characteristics determined in steps 1220, 1230,
1240, and 1250 may be sent to step 1114 of process 1100 (FIG.
11).
[0110] FIG. 13 is a flow chart of illustrative steps for performing
step 1114 of FIG. 11, (i.e., analyzing the set of characteristics
determined in step 1112) in accordance with an embodiment. At step
1310, curve fitting may be performed on the plots of energy values,
energy structural characteristics, signal-to-noise levels, as well
as on any other plots or other data types that may have been
determined in step 1112. The curve fitting may be done using linear
least-squares fitting of data, higher-order interpolative methods,
or any other suitable technique. Parameterization and/or curve
fitting can be performed, for example, by processor 412 (FIG. 4) or
microprocessor 48 (FIG. 2), and may additionally depend on
parameters entered by a user through user inputs 56 (FIG. 2). At
step 1320, signal quality values and associated signal quality
trends may be calculated based on current data obtained from step
1310 as well as from past data (generated in previous iterations of
process 1100 (FIG. 11)) stored in step 1330. Such past data may be
stored in, for example, ROM 52 (FIG. 2) and/or RAM 54 (FIG. 2). At
step 1320, a separate signal quality value and related signal
quality trend may be obtained for each type of available data. For
example, a separate signal quality value and associated signal
quality trend may be determined from each of the signal-to-noise
level plots 810 (FIG. 8) and 820 (FIG. 8). A signal quality value
may be represented by a number from 0 to 100, where a larger number
indicates a higher quality signal and may be determined, for
example, using tabulated figures or merit or through any other
suitable scheme. In addition, signal quality trends may be
determined and stored. A signal quality trend may be determined by
comparing data obtained from step 1310 with past data obtained from
step 1330. A trend may be characterized by a number representing a
rate of increase or rate of decrease in the signal quality, or by
another other suitable statistic. At step 1340, the multiple signal
quality values and signal quality trends determined in step 1320
may be combined to produce a single signal quality value and
associated signal quality trend. Any suitable parsing, combining,
or weighing strategy may be used. For example, maximum-likelihood
techniques may be used to combine data when the prior probability
of a signal quality decrease event is known, and Neyman-Pearson
combining techniques may be used when the prior probability of a
signal quality decrease event is unknown. The overall signal
quality value and associated signal quality trend may be passed to
step 1116 (FIG. 11).
[0111] It will also be understood that the above method may be
implemented using any human-readable or machine-readable
instructions on any suitable system or apparatus, such as those
described herein.
[0112] The foregoing is merely illustrative of the principles of
this disclosure and various modifications can be made by those
skilled in the art without departing from the scope and spirit of
the disclosure. The following claims may also describe various
aspects of this disclosure.
* * * * *